This invention relates generally to communications, and more particularly, to a method for improving data package delivery of wireless communications through dynamic packet sizing.
With the acceptance of wireless communications as a safe and reliable means by which to transfer data, more and more applications are being developed to support this need. These applications can be broadly divided up into two categories, mobile and telemetry.
Mobile devices are those devices where the end device is mobile, such as cellular phones, pagers, and Personal Digital Assistants (PDAs). Telemetry devices are those devices that are in a fixed point, such as an alarm sensor, or a remote meter reading device. Typically, mobile devices are usually operated by a human and are being moved around (such as a device mounted to a vehicle), while telemetry devices are usually unmanned and are fixed.
Wireless DataPacket (WDP) networks, such as ARDIS (Advanced Radio Data Information Systems) and BellSouth Wireless Data""s Mobitex (formerly known as RAM Mobile Data), transfer data between a first device (hereinafter also referred to as tower, base station, or carrier device) and a second device (hereinafter also referred to as remote, modem, or end user device) using data packets. These traditional wireless systems use a fixed packet size to transfer data.
Packets are typically sent using a digital modulation scheme with built in error correction. This error correction is necessary due to the nature of radio frequency (RF) and the effects that the atmosphere has on the signals. When a device receives a data packet, the device will decode the message, using the inverse of the modulation scheme and then calculate a Bit Error Rate (BER), which is a measure of the number of bits that are in error inside the packet. If the BER is low enough, the device can fix the errors in the packet, thus making the packet useable. If the BER is too high, the method of error correction cannot guarantee correct data, thus rendering the packet unusable, forcing it to be discarded. Several negative impacts result if correct data cannot be guaranteed, including negative impacts on the network carrier only, on the application only, and on both the network carrier and the application. Some carriers only bill the customer on xe2x80x9cdeliveredxe2x80x9d packets, meaning correct packets. So, if a packet must be discarded by either the end user device or the tower device, the network carrier cannot bill the customer for that data, thus resulting in a loss of revenue for the carrier.
A negative impact on the end user device is that the information that it was trying to send was not correctly delivered to the tower device, thus requiring it to re-try the transmission of the packet. This is time consuming and unproductive for the end user. If none of the re-tries are successful, then the end user device is not able to get the information to the tower, rendering the application useless. Also, some carriers bill customers for all packets, whether the packets are delivered or discarded, thereby resulting in a higher bill to the customer. A negative impact on both the carrier and the application comes from areas that have highly congested packet traffic. As the traffic increases, the probability of the packet correctly reaching its destination is reduced. It can be shown that the throughput, T, of a pure ALOHA scheme, is T=Rexe2x88x922R where R=xcexxcfx84, xcex is the mean arrival time in packets per second (assuming a Poisson distribution) and T is the average duration of the packet, as described in xe2x80x9cWireless Communications, Principles and Practice,xe2x80x9d Theodore S. Rappaport, published by Prentice Hall PTR, (copyright) 1996, pg. 412. This shows that as the average number of packets is increased or the average duration of the packets is increased, the throughput of the system is decreased. This results in an increased number of packet collisions, which causes the packets to be undeliverable, leading to un-billable revenue activity by the network carrier and a failure of the application for the end user.
A wireless communications system or radio system generates a radio frequency signal that contains information. The system propagates or transmits that signal through the atmosphere with enough strength to be received at the appropriate location. The system preferably performs this function with a high degree of reliability under many different conditions.
Radio signal reception may best be described in terms of probabilities. It is difficult to calculate the actual signal level and effective receiver sensitivity with absolute certainty due to the fluctuations of both signal and noise levels caused by signal reflections. However, it is possible to predict a radio coverage area with a relatively high degree of accuracy.
As shown in FIG. 1, when radio signals operate in an environment free of obstructions (free space), their behavior may be predicted by subtracting radio signal losses from gains. Gains enhance or increase signal strength while losses attenuate or reduce that strength. The signal""s gains minus its losses will determine whether the signal is strong enough for a receiver to recognize the signal.
To determine radio propagation performance in free space, the following gain and loss factors may be considered. Gains that are of most importance are transmitter output power/receiver sensitivity and the transmitter antenna. The RF transmitter is the source of power used by the system. A typical RF network fixed transmitters may operate with about 45 watts of power. A typical subscriber unit device may operate with about xc2xe to one watt of power. This discrepancy is offset by the performance of the receivers and the topology of the area of operation. Base station receivers are typically more sensitive and selective (i.e., they xe2x80x9chear betterxe2x80x9d) than the receivers in subscriber devices.
With regards to the transmitter antenna, a base station antenna converts an electrical signal to an electromagnetic wave that radiates through the atmosphere. Subscriber units detect these waves and translate them back to the original message sent by the host computer. Most antennas radiate electromagnetic waves evenly in all directions. Accordingly, for a typical ground-based system, waves radiate up into the sky and are wasted. For this reason, most networks use antennas that concentrate the radiated signal in the desired directions by reducing signals radiated above the horizon.
Some of the losses that are important in determining radio propagation performance in free space are the transmission line, the free space attenuation, and the subscriber unit antenna. With respect to the transmission line, a typical network""s transmitter/receivers are connected to base station antennas by a transmission line. This line has a loss associated with it that is proportional to its length. Losses associated with free space attenuation result when an electromagnetic wave travels unobstructed through the atmosphere, it loses its power in proportion to the distance that it travels. Several factors cause this attenuation. First, the atmosphere offers resistance to the signal and lowers its strength. Second, as the wave radiates outward, the area it covers increases. Subsequently, the wave""s radiated energy must cover a larger area causing the signal strength to decrease at any particular point. Another loss factor to consider is losses associated with the subscriber unit antenna. Subscriber units typically have internal antennas. For an antenna to provide a gain, or at least have no loss, it must have a certain characteristic length and remain unobstructed by any metallic objects. These features are much easier to design into an exterior antenna. However, an exterior antenna may reduce user convenience by limiting mobility.
Accordingly, in free space, a received signal level can be easily calculated by factoring together gains and losses. If the received signal level is greater than the minimum sensitivity of the receiver, communication will generally occur successfully.
Wireless communications or radio wave communications in the real world are significantly different from the free space propagation model discussed above. In fact, almost all real world factors negatively affect propagation. Although people have been exposed to the use of radio propagation through television and commercial radio all of their lives, there is a considerable amount of confusion about radio propagation when it is applied to a wireless data communications network. Several terms that are commonly used when referring to coverage problems with wireless communications systems are fringe areas and RF dead zones. Fringe areas refer to the areas that are on the xe2x80x9cedgexe2x80x9d of the radio coverage areas. Radio waves do not abruptly stop at the coverage boundaries, but instead, the strength of the radio waves continuously diminishes as the number of obstacles and the distance from the transmitter is increased, as shown in FIG. 2. So as one moves farther away from the coverage area, the signal intensity decreases and the probability of reception likewise decreases. Unlike fringe areas, RF dead zones may occur within the coverage area boundaries. RF dead zones are areas in which communications between a subscriber unit and the network are not possible. The size of an RF dead zone can vary from a few square feet to many city blocks (in some rare instances). RF dead zones are caused by either insufficient signal level (too high a path loss) or too high a noise level in a particular area. RF dead zones are a normal phenomenon and can be expected to occur throughout a coverage area.
The following is a summary of some of the factors that are known to impact data delivery in fringe areas or may cause RF dead zones. Most of these factors obstruct the electromagnetic waves in the atmosphere. The more obstacles it encounters, the weaker the signal will be when it reaches the receiver. The following factors are some of the more significant losses that impede wireless communications and radio propagation.
Signal reflections and the resulting simultaneous reception of multiple signals is the largest obstacle to successful radio communications. This phenomenon is referred to as multi-path reception and is present in virtually every radio system. When a signal is transmitted, an obstacle may absorb or reflect it. In general, the amount of signal absorption or reflection that occurs is dependent upon the type of obstacle encountered and the frequency of the signal. In an urban environment, the signal that arrives at the base station antenna is made up of hundreds of different signals that have all traversed different paths. In virtually every instance, they add up out of phase and have the effect of canceling each other. As these paths are constantly changing, the resultant composite signal level present at the receiver""s antenna is also constantly changing. Moving the receiver""s antenna by just a few inches can dramatically alter the composite received signal level.
Terrain variations cause shadowing of radio waves just as they can shadow the sun""s rays. As terrain variations become more abrupt, so do their effects on radio reception. Radio waves do fill-in behind obstacles, just as there is some light behind an object that is shadowed by the sun. The signal strength in a shadowed area is determined by taking the shadow loss into account. Man-made obstacles such as buildings and bridges make much more abrupt changes than natural obstacles such as hills, valleys, and trees. Because of these abrupt changes, more shadow loss occurs in and around buildings, making radio coverage more difficult.
Another real world propagation variation is atmospheric bending of the radio wave. In space, radio waves travel in straight lines. However, because of the earth""s atmosphere and its changing properties that vary with height, radio waves tend to follow the curvature of the earth. Changing atmospheric conditions such as heavy rain or temperature fluctuations can change radio signal characteristics.
The presence of noise, such as signal and electrical noise, can also dramatically affect the ability of a receiver to pick up a signal. Receivers can have a certain sensitivity when measured in a laboratory environment, but they have diminished effective sensitivity when used in the outside environment. When other signals that are near or on the same frequency are present, radio receivers have a more difficult time selecting the desired signal. These other signals could be caused by atmospheric conditions or man-made devices. Power lines, computers, vehicle electrical systems, and neon lights are just a few of the many noise sources that can interfere with a receiver""s ability to hear a signal. These noise sources can be local to the receiver or may propagate great distances. Just as the desired signal can travel many paths, noise sources can also travel these paths. The end result is that a radio receiver can be subjected to widely varying noise signals at the same time that it is subjected to widely varying desired signals.
Another limitation in the prior art is that conventional wireless communication systems use a fixed data packet size to transfer data. This use of a fixed packet size has several disadvantages. First, it is desirable to transfer data using the largest packet size possible to increase efficiency and to reduce cost. However, in a traditional system using a fixed packet size, the system may not be transferring data at the largest packet size available on the system, thus resulting in an inefficient and more costly transfer of data. At the same time, it is also desirable that the data be successfully transferred. If the system is attempting to transfer data at a relatively large packet size, then the packet having a fixed packet size may not be successfully delivered due to some of the factors known to affect data delivery discussed above, such as system limitations, the nature of the RF signal, or environmental conditions. Again, this fixed packet size leads to inefficiencies in that multiple attempts may be required to successfully transfer the data or the data may not be delivered at all, thereby resulting in a loss of the application and higher costs to the end user and a loss of potential revenue to the carrier.
Although the art of wireless communication and radio wave propagation is well developed, there remain some problems inherent in this technology, particularly with providing a system and method for ensuring the optimum delivery of data packets over a wireless communication system so that the data is sent efficiently and delivered correctly. Therefore, the need exists for a system and method of dynamic packet sizing that overcomes the drawbacks of the prior art.
The present invention is directed to a method for improving the delivery of data between a first signal transmitter/receiver device and a second signal transmitter/receiver device. The method comprises the steps of dynamically sizing the data into at least one data packet having a data packet size that is no greater than an operational maximum data packet size capable of being transferred between the first wireless signal transmitter/receiver device and the second wireless signal transmitter/receiver device, and transferring the at least one data packet from the first signal transmitter/receiver device to the second signal transmitter/receiver device until all data has been delivered.
According to one aspect of the present invention, the step of dynamically sizing the data into the at least one data packet further comprises the steps of: determining an operational maximum allowable packet size capable of being transferred between the first signal transmitter/receiver device and the second signal transmitter/receiver device; and setting the data packet size of the at least one data packet to the data packet size no greater than the operational maximum allowable data packet size.
In accordance with an aspect of the present invention, the step of dynamically sizing the data into the at least one data packet further comprises the steps of reading a stored data packet size from a memory at the second signal transmitter/receiver device; determining an operational maximum allowable data packet size; increasing the data packet size and storing it in the memory, if the stored data packet size is less than the operational maximum allowable packet size; decreasing the data packet size and storing it in the memory, if the stored data packet size is greater than the operational maximum allowable packet size; and packaging the data into at least one data packet, each data packet having the data packet size.
In accordance with a further aspect of the present invention, the step of transferring further comprises the steps of transmitting the at least one data packet, and receiving the at least one data packet between the first signal transmitter/receiver device and the second signal transmitter/receiver device.
In accordance with a further aspect of the present invention, the step of reading the packet size further comprises the steps of sending a request signal from the first signal transmitter/receiver device to the second signal transmitter/receiver device for the second transmitter/receiver device to read a stored packet size from a memory of the second transmitter/receiver device, and reading the stored data packet size.
In accordance with a further aspect of the present invention, the step of determining an operational maximum allowable packet size further comprises the steps of transmitting a signal having the stored data packet size between the first signal transmitter/receiver device and the second signal transmitter/receiver device, determining whether the data packet is successfully transferred, and sizing the data packet size by increasing or decreasing the stored data packet size until a successful transfer of data is determined.
In a further embodiment within the scope of the present invention, a system for improving data packet delivery of wireless communications comprises a first wireless signal transmitter/receiver device having a transmitter, a receiver, a memory, and a microprocessor, the first signal transmitter/receiver device for transmitting a data packet; a second wireless signal transmitter/receiver device having a transmitter, a receiver, a memory, and a microprocessor, the second signal transmitter/receiver device for receiving the data packet; and means for dynamically sizing a data packet for improving data packet delivery between the first signal transmitter/receiver device and the second signal transmitter/receiver device.
According to further aspects of the present invention, the means for dynamic packet sizing further comprises means for dynamically segmenting data and encapsulating the segmented data into at least one data packet having a data packet size that is an operational maximum data packet size. The operational maximum data packet size is a data packet size substantially close to and not greater than a maximum data packet size capable of being successfully transferred between the first signal transmitter/receiver device and the second signal transmitter/receiver device.
Another embodiment within the scope of this invention includes a system for improving data packet delivery of wireless communications between a first signal transmitter/receiver device and a second signal transmitter/receiver device, comprising means for reading a stored data packet size; means for determining an operational maximum data packet size; means for increasing and storing the data packet size if the stored data packet size is less than the operational maximum data packet size; means for decreasing and storing the data packet size if the stored data packet size is greater than the operational maximum data packet size; means for packaging data into at least one data packet having the operational maximum data packet size; means for transmitting the data packet between the first signal transmitter/receiver device and the second signal transmitter/receiver device; and means for receiving the data packet between the first signal transmitter/receiver device and the second signal transmitter/receiver device.